Cask::Audit: Align user agents with livecheck

The `#page_headers` and `#page_content` methods in
`Livecheck::Strategy` will fetch a URL using our default user agent
but if the request fails it will retry with the `:browser` user agent.
[For context, it was added as an interim measure to make URLs work
that require a different user agent but I aim to remove it in the
future in favor of specifying the user agent in a `livecheck` block
(so we don't make unnecessary requests that we know will fail).]

`Cask::Audit#audit_livecheck_https_availability` checks the
`livecheck` block URL but it only does so using our default user
agent (i.e., it calls `#validate_url_for_https_availability` which
calls `Utils::Curl#curl_check_http_content` which has a `user_agents:
[:default]` parameter). Due to this behavioral mismatch, it's possible
for a `livecheck` block to work but for this cask audit to fail.

This addresses the issue by adding `user_agents: [:default, :browser]`
to the arguments the audit uses, which aligns its behavior with
livecheck's.

Library/Homebrew/cask/audit.rb

@@ -839,7 +839,8 @@ module Cask
 
       validate_url_for_https_availability(
         url, "livecheck URL",
-        check_content: true
+        check_content: true,
+        user_agents:   [:default, :browser]
       )
     end